The present invention generally relates to the conversion of organic lignocellulosic materials (biomass) into useful fuels (biofuels), and more particularly to a system and process capable of controlling the production, including the elimination, of carbon char in the ash produced by a biomass gasification or pyrolysis reactor.
Synthesis gas (syngas) is a gas mixture containing carbon monoxide (CO) and hydrogen gas (H2) produced by the thermochemical conversion of carbonaceous materials, such as coal, petroleum, and biomass materials. Though having a lower energy density than natural gas, syngas is suitable for use as a fuel source for a variety of applications, including but not limited to gas turbines and automotive internal combustion engines. Syngas can also be used to produce methanol and hydrogen, or converted via the Fischer-Tropsch process to produce a synthetic petroleum substitute.
The use of syngas as a fuel is more efficient than direct combustion of the original biomass because more of the energy contained in the biomass is extracted by the conversion process, known as gasification. Within a typical biomass gasifier, a carbonaceous material is combusted in an atmosphere where the oxygen content is below the stoichiometric limit at which complete combustion can occur. This oxygen-starved combustion of carbonaceous material releases volatiles and heat. In the case of dry feedstock, the oxygen-starved combustion also produces an ash that has a carbon-rich content, referred to as char or, more recently, biochar. The combustion heat raises the temperature of the non-combusted carbonaceous material within the ash, causing it to pyrolyze and release flammable volatiles such as carbon monoxide (CO) and hydrogen (H2). Steam (H2O) may be injected to react with the char to produce more CO and H2. Depending on process temperatures, methane (CH4) and hydrocarbon molecules having a greater number of carbon atoms may also be produced. Consequently, biomass gasification processes employ substoichiometric quantities of oxygen or air to combust a portion of the biomass and through pyrolysis, and the optional injection of steam, produce heat (energy) and a syngas that contains a blend of flammable volatiles.
Plant biomass is composed largely of carbon and oxygen, with smaller amounts of hydrogen. As noted above, when dry feedstock materials of plant biomass are gasified, there is an excess of carbon and the resulting char causes the ash to have a darker appearance (black ash). On the other hand, moist plant biomass contains relatively greater amounts of hydrogen and oxygen, with the result that the gasification consumes essentially all the carbon of a moist biomass, and the resulting ash has a white/gray appearance. The syngas produced by gasifying a moist biomass will also contain a component of water vapor, which generally must be removed. Depending on the particular application, it may be more desirable to favor the production of syngas (with the result that a white/gray ash is produced) or the production of char (a black ash that contains excess carbon), in which case it may be desirable to control the carbon content of the char to levels favorable for ash post-processing.
Control systems adapted for use in biomass gasification processes are well known. As reported in U.S. Published Patent Application 2010/0275514 and U.S. patent application Ser. No. 12/760,241 (filed Apr. 14, 2010), such systems are often closed loop systems that use feedback control that rely on the outputs of various measurement devices.